Alice (a wizard) & Bob (class unspecified), are planning an elaborate prank on Carol (a cleric). Alice casts Feign Death on Bob, and then wails about how he has been slain by a horrible monster/curse/whatever. Carol, who is, for some reason, friends with these jerks, comes running. Carol does not routinely prepare resurrection spells; instead, she casts Gentle Repose on Bob to preserve him for tomorrow, when she will prepare an appropriate spell to bring him back to life.
Is Carol aware that Bob is not a valid target for the Gentle Repose spell (since he isn’t actually dead), either before or after attempting to cast the spell?
From the PHB, p140:
You touch a willing creature and put it into a cataleptic state that is indistinguishable from death. For the spell’s duration… the target appears dead to all outward inspection and to spells used to determine the target’s status. ….
From the PHB, p215:
You touch a corpse or other remains. For the duration, the target is protected from decay and can’t become undead. …